This project examines the effects of two variables on the development of class inclusion reasoning in young children: whether the problem is given in the context of other problems involving the comparison of classes, and whether the child is asked to explain his response. Children aged 5, 7, and 9 will be shown arrays containing 2 superordinate classes, each with 2 subordinate elements (e.g., animals, consisting of dogs and cats, and children, consisting of boys and girls). Different groups will be asked inclusion questions involving 1 of the superordinate classes, comparison questions referring to 1 subordinate element (i.e., dogs) and 1 superordinate element (children), or a combination of inclusion and comparison questions. A second group of children will be given the class inclusion questions only, but will not be asked to rationalize their responses. Both answers and accompanying explanations will be examined to determine the extent to which context effects the strategy children apply to class comparison problems.